Apparatus of driving light source for display device

ABSTRACT

An apparatus of driving a lamp for a display device is provided. The driving apparatus includes an inverter ( 920 ), a lamp current sensor ( 940 ), and an inverter controller ( 930 ). The lamp current sensor ( 940 )senses a current flowing in the lamp and output a feedback signal having a magnitude depending on the sensed current. The inverter controller ( 930 ) compares a dimming control signal from an external device with the feedback signal and controls the inverter ( 920 ) based on the comparison. The inverter ( 920 ) includes a transformer (T 1 ) for applying a lamp drive voltage to a lamp for turning on or off the lamp and a voltage sensor ( 928 ) sensing the lamp drive voltage. The inverter ( 920 ) adjusts a turns ratio of the transformer in accordance with the sensed lamp drive voltage.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an apparatus of driving light sourcefor a display device such as a liquid crystal display.

(b) Description of the Related Art

Display devices used for monitors of computers and television setsinclude self-emitting displays such as light emitting diodes (LEDs),electroluminescences (ELs), vacuum fluorescent displays (VFDs), fieldemission displays (FEDs) and plasma panel displays (PDPs) andnon-emitting displays such liquid crystal displays (LCDs) requiringlight source.

An LCD includes two panels provided with field-generating electrodes anda liquid crystal (LC) layer with dielectric anisotropy interposedtherebetween. The field-generating electrodes supplied with electricvoltages generate electric field in the liquid crystal layer, and thetransmittance of light passing through the panels varies depending onthe strength of the applied field, which can be controlled by theapplied voltages. Accordingly, desired images are obtained by adjustingthe applied voltages.

The light may be emitted from a light source equipped in the LCD or maybe natural light When using the equipped light source, the totalbrightness of the LCD screen is usually adjusted by regulating the ratioof on and off times of the light source or regulating the currentthrough the light source.

A light device for an LCD, i.e., a backlight unit usually includes aplurality of fluorescent lamps as a light source and an inverter fordriving the lamps, which includes a transformer with a boosting voltagetypically determined based on the turns ratio. The inverter converts aDC (direct current) input voltage from an external device into an AC(alternating current) voltage, and then applies the voltage boosted bythe transformer to the lamps to turn on the lamps and to control thebrightness of the lamps in response to a luminance control signal.Furthermore, the inverter detects a voltage related to a total currentflowing in the lamps and controls the voltage applied to the lamps onthe basis of the detected voltage.

Since the lamp of the backlight unit is supplied with a high voltage forstable lighting operation in case of initiation or under a lowtemperature, the turns ratio of the transformer is set to be high togenerate the high voltage.

However, since the initiation or the low temperature condition ismaintained for a very short time, the lamp is not required to besupplied with such a high voltage for most time. That is, after ashort-time application of the high voltage to the lamp for an initiatingoperation or a low temperature lighting operation, a low voltage issufficient for driving the lamp.

In a typical backlight unit, the design of the inverter of the backlightunit focuses on the initiating condition or the low temperaturecondition rather than a normally operating state after ignition of thelamp. For this purpose, the ratio of the transformer is set to be high,which continuously applies high voltage to the lamp even in thestabilized state to cause unnecessary power consumption and decrease inoperation efficiency.

Particularly, the efficient power consumption is very important for adevice with a battery having a limited capacity such as a portablecomputer.

SUMMARY OF THE INVENTION

A motivation of the present invention is to improve the power efficiencyof a light device.

An apparatus of driving a lamp for a display device is provided, theapparatus includes: an inverter including a transformer for applying alamp drive voltage to a lamp for turning on or off the lamp and avoltage sensor sensing the lamp drive voltage; a lamp current sensorsensing a current flowing in the lamp and output a feedback signalhaving a magnitude depending on the sensed current; and an invertercontroller comparing a dimming control signal from an external devicewith the feedback signal and controlling the inverter based on thecomparison, wherein the inverter adjusts a turns ratio of thetransformer in accordance with the sensed lamp drive voltage.

It is preferable that the turns ratio of the transformer is adjusted toa first value when the sensed lamp drive voltage is higher than apredetermined voltage, and to a second value lower than the first valuewhen the sensed lamp drive voltage is less than the predeterminedvoltage.

The inverter controller operates depending on an enable signal having aplurality of states from an external device, the inverter furthercomprises a differential circuit supplied with the enable signal, andthe turns ratio of the transformer has the first value when the enablesignal is in the first state.

The inverter may further includes: a driving operation selector havingan output value in accordance with outputs of the inverter controllerand the differential circuit; a driving unit adjusting the turns ratioof the transformer based on the outputs of the driving operationselector and the inverter controller; and a voltage divider connectedbetween the transformer and the voltage sensor, and making an outputvoltage of the transformer resonate and dividing the output voltage.

The inverter controller may output a first signal and a second signal,and the driving operation selector may include: a first OR gate suppliedwith the first output signal of the inverter controller; a second ORgate supplied with the first output signal of the inverter controllerand the output of the differential circuit; a first AND gate suppliedwith the second output signal of the inverter controller and the outputof the differential circuit; a second AND gate supplied with the secondoutput signal of the inverter controller; and an inverter receiving theoutput of the differential circuit, and generating an output signal tobe applied to the first OR gate and the second AND gate.

The inverter controller may further outputs a third signal and a fourthsignal, the transformer includes a primary coil having an input terminaland first and second terminals and a secondary coil, and the drivingunit includes: a first driving circuit supplied with the third andfourth output signals of the inverter controller and generating anoutput signal to be applied to the input terminal of the primary coil ofthe transformer; a second driving circuit supplied with the outputs fromthe first OR gate and the first AND gate and generating an output signalto be applied to the first output terminal of the primary coil of thetransformer; and a third driving circuit supplied with the outputs fromthe second OR gate and the second AND gate and generating an outputsignal to be applied to the second output terminal of the primary coilof the transformer.

The number of turns for the first output terminal of the transformer isless than the number of turns for the second output terminal of thetransformer.

Preferably, the driving operation selector makes the second drivingcircuit select the first output terminal of the transformer to beactivated when the sensed lamp drive voltage is higher than thepredetermined voltage, and makes the third driving circuit select thesecond output terminal of the transformer to be activated when thesensed lamp drive voltage sensor is smaller than the predeterminedvoltage.

The voltage divider may include first and second capacitors connected inseries between the transformer and a ground voltage, and a commonterminal of the first and the second capacitors is preferably connectedto the voltage sensor.

The voltage sensor may include: a diode having an anode connected to thecommon terminal of the first and the second capacitors and a cathodeconnected to the differential circuit; a resistor connected between thecathode of the diode and a ground; and a capacitor connected between thecathode of the diode and the ground.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of the present invention will become moreapparent by describing preferred embodiments thereof in detail withreference to the accompanying drawings in which:

FIG. 1 is a block diagram of an LCD according to an embodiment of thepresent invention;

FIG. 2 is an exploded perspective view of an LCD according to anembodiment of the present invention;

FIG. 3 is an equivalent circuit diagram of a pixel of an LCD accordingto an embodiment of the present invention;

FIG. 4 is a circuit diagram of an inverter according to an embodiment ofthe present invention;

FIGS. 5A and 5B are equivalent circuit diagrams of the inverter shown inFIG. 4 when sensed voltages are high and low, respectively; and

FIG. 6 is a graph illustrating a sensed voltage as function of timeaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Like numerals refer to like elementsthroughout.

In the drawings, the thickness of layers and regions are exaggerated forclarity. Like numerals refer to like elements throughout It will beunderstood that when an element such as a layer, region or substrate isreferred to as being “on” another element, it can be directly on theother element or intervening elements may also be present In contrast,when an element is referred to as being “directly on” another element,there are no intervening elements present.

Then, liquid crystal displays according to embodiments of the presentinvention will be described with reference to the drawings.

FIG. 1 is a block diagram of an LCD according to an embodiment of thepresent invention, FIG. 2 is an exploded perspective view of an LCDaccording to an embodiment of the present invention, and FIG. 3 is anequivalent circuit diagram of a pixel of an LCD according to anembodiment of the present invention.

Referring to FIG. 1, an LCD according to an embodiment of the presentinvention includes a LC panel assembly 300, a gate driver 400 and a datadriver 500 which are connected to the panel assembly 300, a gray voltagegenerator 800 connected to the data driver 500, a lamp unit 910 forilluminating the panel assembly 300, an inverter 920 connected to thelamp unit 910, a lamp current sensor 940 connected to the lamp unit 910,a inverter controller 930 connected to the lamp current sensor 940 andthe inverter 920, and a signal controller 600 controlling the aboveelements.

In structural view, the LCD according to an embodiment of the presentinvention includes a LC module 350 including a display unit 330 and abacklight unit 340, and a pair of front and rear cases 361 and 362containing the LC module 350 as shown in FIG. 2.

The display unit 330 includes the LC panel assembly 300, a plurality ofgate flexible printed circuit (FPC) films 410 and a plurality of dataFPC films 510 attached to the LC panel assembly 300, and a gate printedcircuit board (PCB) 450 and a data PCB 550 attached to the associatedFPC films 410 and 510, respectively.

The LC panel assembly 300, in structural view shown in FIGS. 2 and 3,includes a lower panel 100, an upper panel 200 and a liquid crystallayer 3 interposed therebetween while it includes a plurality of displaysignal lines G₁-G_(n) and D₁-D_(m) and a plurality of pixels connectedthereto and arranged substantially in a matrix in circuital view shownin FIGS. 1 and 3.

The display signal lines G₁-G_(n) and D₁-D_(m) are provided on the lowerpanel 100 and include a plurality of gate lines G₁-G_(n) transmittinggate signals (called scanning signals) and a plurality of data linesD₁-D_(m) transmitting data signals. The gate lines G₁-G_(n) extendsubstantially in a row direction and are substantially parallel to eachother, while the data lines D₁-D_(m) extend substantially in a columndirection and are substantially parallel to each other.

Each pixel includes a switching element Q connected to the displaysignal lines G₁-G_(n) and D₁-D_(m), and an LC capacitor C_(LC) and astorage capacitor C_(ST) that are connected to the switching element Q.The storage capacitor C_(ST) may be omitted if unnecessary.

The switching element Q such as a TFT is provided on the lower panel 100and has three terminals: a control terminal connected to one of the gatelines G₁-G_(n); an input terminal connected to one of the data linesD₁-D_(m); and an output terminal connected to the LC capacitor C_(LC)and the storage capacitor C_(ST).

The LC capacitor C_(LC) includes a pixel electrode 190 on the lowerpanel 100, a common electrode 270 on the upper panel 200, and the LClayer 3 as a dielectric between the electrodes 190 and 270. The pixelelectrode 190 is connected to the switching element Q, and the commonelectrode 270 covers the entire surface of the upper panel 100 and issupplied with a common voltage Vcom. Alternatively, both the pixelelectrode 190 and the common electrode 270, which have shapes of bars orstripes, are provided on the lower panel 100.

The storage capacitor C_(ST) is an auxiliary capacitor for the LCcapacitor C_(LC). The storage capacitor C_(ST) includes the pixelelectrode 190 and a separate signal line (not shown), which is providedon the lower panel 100, overlaps the pixel electrode 190 via aninsulator, and is supplied with a predetermined voltage such as thecommon voltage Vcom. Alternatively, the storage capacitor C_(ST)includes the pixel electrode 190 and an adjacent gate line called aprevious gate line, which overlaps the pixel electrode 190 via aninsulator.

For color display, each pixel represent its own color by providing oneof a plurality of red, green and blue color filters 230 in an areaoccupied by the pixel electrode 190. The color filter 230 shown in FIG.3 is provided in the corresponding area of the upper panel 200.Alternatively, the color filter 230 is provided on or under the pixelelectrode 190 on the lower panel 100.

Referring to FIG. 2, the backlight unit 340 includes 340 includes aplurality of lamps 341 disposed behind the LC panel assembly 300, alight guide 342 and a plurality of optical sheets 343 disposed betweenthe panel assembly 300 and the lamps 341 and guiding and diffusing lightfrom the lamps 341 to the panel assembly 300, and a reflector 344disposed under the lamps 341 and reflecting the light from the lamps 341toward the panel assembly 300.

The lamps 341 preferably include fluorescent lamps such as CCFL (coldcathode fluorescent lamp) and EEFL (external electrode fluorescentlamp). An LED is another example of the lamp 341.

The inverter 920, the lamp current sensor 940, and the invertercontroller 930 may be mounted on a stand-alone inverter PCB (not shown)or mounted on the gate PCB 450 or the data PCB 550.

A pair of polarizers (not shown) polarizing the light from the lamps 341are attached on the outer surfaces of the panels 100 and 200 of thepanel assembly 300.

Referring to FIGS. 1 and 2, the gray voltage generator 800 generates twosets of a plurality of gray voltages related to the transmittance of thepixels and is provided on the data PCB 550. The gray voltages in one sethave a positive polarity with respect to the common voltage Vcom, whilethose in the other set have a negative polarity with respect to thecommon voltage Vcom.

The gate driver 400 preferably includes a plurality of integratedcircuit (IC) chips mounted on the respective gate FPC films 410. Thegate driver 400 is connected to the gate lines G₁-G_(n) of the panelassembly 300 and synthesizes the gate-on voltage Von and the gate offvoltage Voff from the driving voltage generator 700 to generate gatesignals for application to the gate lines G₁-G_(n).

The data driver 500 preferably includes a plurality of IC chips mountedon the respective data FPC films 510. The data driver 500 is connectedto the data lines D₁-D_(m) of the panel assembly 300 and applies datavoltages selected from the gray voltages supplied from the gray voltagegenerator 800 to the data lines D₁-D_(m).

According to another embodiment of the present invention, the IC chipsof the gate driver 400 and/or the data driver 500 are mounted on thelower panel 100, while one or both of the drivers 400 and 500 areincorporated along with other elements into the lower panel 100according to still another embodiment. The gate PCB 450 and/or the gateFPC films 410 may be omitted in both cases.

The signal controller 600 controlling the drivers 400 and 500, etc. isprovided on the data PCB 550 or the gate PCB 450.

Now, the operation of the LCD will be described in detail.

The signal controller 600 is supplied with RGB image signals R, G and Band input control signals controlling the display thereof such as avertical synchronization signal Vsync, a horizontal synchronizationsignal Hsync, a main clock MCLK, and a data enable signal DE, from anexternal graphic controller (not shown). After generating gate controlsignals CONT1 and data control signals CONT2 and processing the imagesignals R, G and B suitable for the operation of the panel assembly 300on the basis of the input control signals and the input image signals R,G and B, the signal controller 600 provides the gate control signalsCONT1 for the gate driver 400, and the processed image signals R′, G′and B′ and the data control signals CONT2 for the data driver 500.

The gate control signals CONT1 include a vertical synchronization startsignal STV for informing of start of a frame, a gate clock signal CPVfor controlling the output time of the gate-on voltage Von, and anoutput enable signal OE for defining the width of the gate-on voltageVon. The data control signals CONT2 include a horizontal synchronizationstart signal STH for informing of start of a horizontal period, a loadsignal LOAD or TP for instructing to apply the appropriate data voltagesto the data lines D₁-D_(m), an inversion control signal RVS forreversing the polarity of the data voltages (with respect to the commonvoltage Vcom) and a data clock signal HCLK.

The data driver 500 receives a packet of the image data R′, G′ and B′for a pixel row from the signal controller 600 and converts the imagedata R′, G′ and B′ into the analogue data voltages selected from thegray voltages supplied from the gray voltage generator 800 in responseto the data control signals CONT2 from the signal controller 600.

Responsive to the gate control signals CONT1 from the signals controller600, the gate driver 400 applies the gate-on voltage Von to the gateline G₁-G_(n), thereby turning on the switching elements Q connectedthereto.

The data driver 500 applies the data voltages to the corresponding datalines D₁-D_(m) for a turn-on time of the switching elements Q (which iscalled “one horizontal period” or “1H” and equals to one periods of thehorizontal synchronization signal Hsync, the data enable signal DE, andthe gate clock signal CPV). Then, the data voltages in turn are suppliedto the corresponding pixels via the turned-on switching elements Q.

The difference between the data voltage and the common voltage Vcomapplied to a pixel is expressed as a charged voltage of the LC capacitorC_(LC), i.e., a pixel voltage. The liquid crystal molecules haveorientations depending on the magnitude of the pixel voltage and theorientations determine the polarization of light passing through the LCcapacitor C_(LC). The polarizers convert the light polarization into thelight transmittance.

By repeating this procedure, all gate lines G₁-G_(n) are sequentiallysupplied with the gate-on voltage Von during a frame, thereby applyingthe data voltages to all pixels. When the next frame starts afterfinishing one frame, the inversion control signal RVS applied to thedata driver 500 is controlled such that the polarity of the datavoltages is reversed (which is called “frame inversion”). The inversioncontrol signal RVS may be also controlled such that the polarity of thedata voltages flowing in a data line in one frame are reversed (which iscalled “line inversion”), or the polarity of the data voltages in onepacket are reversed (which is called “dot inversion”).

The inverter 920 converts a DC voltage VIN into an AC voltage, booststhe AC voltage and applies the boosted AC voltage to the lamp unit 910in response to an inverter control signal from the inverter controller930.

The lamp current sensor 940 senses a current flowing into the lamp unit940 to provide a voltage related to the sensed current as a feedbacksignal VFB for the inverter controller 930.

The inverter controller 930 starts operation thereof upon application ofan enable signal EN from an external device for controlling the inverter920. The inverter controller 930 compares the feedback signal VFB fromthe lamp current sensor 940 with a dimming control voltage V_(dim), and,based on the comparison result, supplies pulse-width-modulated (PWMed)control signals for controlling the inverter 920 to the inverter 920. Atthis time, when the feedback signal VFB is less than the dimming controlvoltage V_(dim), the inverter controller 930 increases the pulse widthof the control signals for applying higher voltage to the lamp unit 910.On the contrary, when the feedback signal VFB is higher than the dimmingcontrol voltage V_(dim), the inverter controller 930 decreases the pulsewidth of the control signals for applying lower voltage to the lamp unit910. Accordingly, the current flowing in the lamp unit 910 is alwaysuniform.

The dimming control voltage V_(dim) may be directly inputted from aseparate input device which is adjustable by a user, or from the signalcontroller 600. In addition, the enable signal EN for starting thebacklight unit may be supplied from a separate control device providedon an external device.

Then, the operations of the inverter controller 930 and the inverter 920will be described in detail with reference to FIGS. 4 to 6. Although onelamp is shown for descriptive purpose, but a plurality of lampsconnected in parallel may be provided.

FIG. 4 is a circuit diagram of an inverter according to an embodiment ofthe present invention. FIGS. 5A and 5B are equivalent circuit diagramsof the driving apparatus when sensed voltages are high and low,respectively, and FIG. 6 is a graph showing a sensed voltage as functionof time according to an embodiment of the present invention.

As shown in FIG. 4, an inverter 920 according to an embodiment of thepresent invention includes a delay unit 921 receiving an enable signalEN, a driving operation selector 922 connected to the invertercontroller 930, a driving unit 923 connected to the inverter controller930 and the driving operation selector 922, a transformer T1 connectedto the driving unit 923, a voltage divider 927 connected to thetransformer T1, and a voltage sensor 928 connected between the voltagedivider 927 and the delay unit 921.

The delay unit 921 includes a combination of a resistor R1 and acapacitor C connected in parallel and a resistor R2 connected thereto.The combination of a resistor R1 and a capacitor C receives the enablesignal while the resistor R2 is supplied with a driving operation selectsignal V_(sel) from the voltage sensor 928. The output terminal of thedelay unit 921 is a node between the resistors R1 and R2.

The driving operation selector 922 includes a plurality of logic gates,one inverter INV1, two OR gates OR1 and OR2, and two AND gates AND1 andAND2. An input of the inverter INV1 is the output of the delay unit 921.Two inputs of the OR gate OR1 are connected to an output terminal OUT3of the inverter controller 930 and an output terminal of the inverterINV1, while two inputs of the OR gate OR2 are connected to an outputterminal OUT3 of the inverter controller 930 and the output terminal ofthe delay unit 921. Two inputs of the AND gate AND1 are connected to anoutput terminal OUT1 of the inverter controller 930 and the outputterminal of the delay unit 921, while two inputs of the AND gate AND 2are connected to an output terminal OUT4 of the inverter controller 930and the output terminal of the inverter INV1.

The driving unit 923 includes a plurality of current drive units924-926. Each current drive unit 924-926 has a pair of transistors Tr1and Tr2, Tr3 and TR4, or Tr5 and Tr6 connected in series between asupply voltage VIN and a ground voltage, and the nodes between thetransistors Tr1 and Tr2, Tr3 and TR4, and Tr5 and Tr6 are outputterminals of the respective current drive units 924-926.

Gates of the transistors Tr1 and Tr2 of the current drive unit 924 areconnected to the output terminals OUT1 and OUT2 of the invertercontroller 930, respectively, gates of the transistors Tr3 and Tr4 ofthe current drive unit 925 are connected to the output terminals of theOR gate OR1 and the AND gate AND1 of the driving operation selector 922,respectively, and gates of the transistors Tr5 and Tr6 of the currentdrive unit 926 are connected to the output terminals of the OR gate OR2and the AND gate AND2 of the driving operation selector 922,respectively.

The output terminals of the current drive units 924-926 are connected toan input terminal, a first output terminal A, and a second outputterminal B of a primary coil L1 of the transformer T1, respectively.

The voltage divider 927 includes a pair of capacitors C2 and C3connected in series. The capacitors C2 and C3 are connected across asecondary coil L2 of the transformer T1. The capacitor C3 is grounded.

The voltage sensor 928 includes a diode D3 connected from a commonterminal of the capacitors C2 and C3 in a forward direction, and aresistor R4 and a capacitor C4 connected in parallel between an outputterminal of the diode D3 and the ground voltage. An output of the diodeD3 is applied as the drive select signal V_(sel) to the delay unit 921.

The first output terminal A of the primary coil L1 of the transformer T1has the turns number less than that of the second output terminal B ofthe primary coil L1, and the secondary coil L2 of the transformer T2 isconnected across the lamp LAM1 of the lamp unit 10.

According to an embodiment of the present invention, the transistorsTr1-Tr6 of the current drive unit 924 to 926 are metal oxidesemiconductor field effect transistors (MOSFET). The transistors Tr1,Tr3 and Tr5 among them are p-channel transistors, while the remaindertransistors Tr2, Tr4 and Tr6 are n-channel transistors However, types ofthe transistors may be changed.

The lamp current sensor 940 includes a pair of diodes D2 and D3, and aresistor R3. The diodes D2 and D3 are connected from the lamp LAM1 in aforward direction and in a reverse direction, respectively, and theresistor R3 is connected between the diode D2 and the ground voltage.The feedback control signal VFB is outputted from a common terminal ofthe diode D2 and the resistor R3.

The operations of the inverter 920 will be described in detail.

For stating the backlight device, when the enable signal EN applied tothe delay unit 921 and the inverter controller 930 is changed from a lowstate into a high state, the inverter controller 930 outputs controlsignals with appropriate states via the output terminals OUT1 to OUT4 tooperate the inverter 920. That is, the inverter controller 930 suppliesa signal in a high state and a signal in a low state for the gates ofthe transistors Tr1 and Tr2 of the current drive unit 924 through theoutput terminals OUT1 and OUT2 thereof, respectively, to turn on thetransistors Tr1 and Tr2.

In addition, the inverter controller 930 supplies a signal in a lowstate for the OR gates OR1 and OR2 through the output terminal OUT3thereof and a signal in a high state for one terminals of the AND gatesAND1 and AND2 through the output terminal OUT4 thereof, respectively.

The enable signal in the high state is applied to the delay unit 921.Then, since the resistor R1 and the capacitor C1 of the delay unit 921function as a differential circuit outputting a high-level signal whenthe enable signal EN is changed from the low state into the high state,the OR gate OR1 and the AND gate AND2 of the driving operation selector922 receive high inputs. Furthermore, because an input into the inverterINV1 is in the high state, the inverter INV1 outputs low-level signal tobe input into the OR gate OR1 and the AND gate AND2.

Because the outputs from the OR gate OR1 and the AND gate AND1 of thedriving operation selector 922 are in the low state and in the highstate, respectively, the transistors Tr3 and Tr4 of the current driveunit 925 are turned on. However, because the outputs from the OR gateOR2 and the AND gate AND2 of the driving operation selector 922 are highand low, respectively, the transistors Tr5 and Tr6 of the current driveunit 926 are turned off.

By the operations of the current drive units 925 and 926, the inputterminal and the output terminal A1 of the primary coil L1 of thetransformer T1 are selected to form an equivalent circuit as shown inFIG. 5A. The transformer T1 transforms an alternating voltage applied tothe primary coil L1 and applies the transformed alternating voltage tothe voltage divider 927, based on the turns ratio of the transformer T1defined by the selection of the input terminal and the first outputterminal A.

Because the number of turns for the first output terminal A is less thanthat for the second output terminal B, the applied voltage for theselection of the first output terminal A is higher than that for theselection of the second output terminal B. The transformed high voltageresonates and is divided by the voltage divider 927.

The transformed high voltage is applied to the lamp LAM1 to turn on thelamp LAM1, and is divided to be applied to the voltage sensor 928.

By an operation characteristic of the lamp LAM 1, when a current flowsthrough the lamp LAM1 after ignition of the lamp LAM1, the temperatureof the lamp LAM1 increases, and thus the impedance of the lamp LAM1decreases. Since the inverter controller 930 controls the inverter 920to apply a uniform current to the lamp LAM1 by using the feedback signalVFB from the lamp current sensor 940 and the dimming control signalV_(dim) the magnitude of the driving operation select signal V_(sel)from the voltage sensor 928 is gradually decreased with time as thetemperature of the lamp LAM1 increases.

When the magnitude of the driving operation select signal V_(sel)applied to the delay unit 921 is less than about a half of the supplyvoltage VIN, the driving operation selector 922 determines the drivingoperation select signal V_(sel) in a low state and vice versa. Forexample, in a case that the supply voltage VIN is 5V, the drivingoperation select signal V_(sel) is determined to be high when itsmagnitude is higher than 2.5V while it is determined to be low when itsmagnitude is less than 2.5V.

According to the operation characteristic, the magnitude of the drivingoperation select signal V_(sel) from the voltage sensor 928 is graduallydecreased with time as shown in FIG. 6, to reach 2.5V at a time t1.Accordingly, the output signals of the OR gate OR1 and the AND gate AND1are changed into the high state and the low state, respectively, and theoutput signals of the OR gate OR2 and the AND gate AND2 are changed intothe low state and the high state, respectively. Thus, the transistorsTr3 and Tr4 of the current drive unit 925 are turned off, while thetransistors Tr5 and Tr6 of the current drive unit 926 are turned on, andthus the input terminal and the second output terminal B are selected.An equivalent circuit thereof is shown in FIG. 5B. That is, the outputterminal of the primary coil L1 is changed from the first outputterminal A into the second output terminal B.

The transformer T1 transforms the alternating voltage applied to theprimary coil L1 in accordance with the turns ratio of the transformer T1defined by the selection of the input terminal and the second outputterminal B and applies the transformed alternating voltage to thevoltage divider 927.

As above described, because the number of turns for the first outputterminal A of the primary coil L1 is less than that for the secondoutput terminal B of the primary coil L1, the turns ratio of thetransformer T1 is less than the ratio defined on the initiatingcondition. Thus, the transformed alternating voltage applied to the lampLAM1 is decreased.

The operations of the inverter controller 930 and the inverter 920 areperformed along with the feedback control operation of the inverter 920using the feedback control signal VFB from the lamp current sensor 940and the dimming control voltage V_(dim). That is, a voltagecorresponding to a current rectified by the diode D2 is applied as thefeedback control signal VFB to the inverter controller 930. Then, theinverter controller 930 compares the driving operation select signalV_(sel) with the feedback control signal VPB, and the pulse widths ofthe control signals applied to the driving operation selector 922 andthe driving unit 923 are adjusted based on the difference of the signalsV_(sel) and VFB such that the uniform current is flowing in the lampLAM1.

According to an embodiment of the present invention, the voltage acrossthe lamp LAM1 is sensed and the turns ratio of the transformer T1 ischanged according to the sensed voltage to adjust the transformedalternating voltage from the transformer T1. The turns ratio of thetransformer T1 is increased under the initiating condition or the lowtemperature condition, while it is decreased when the lamp isstabilized, thereby improving power efficiency of the backlight unit.

Although preferred embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptsherein taught which may appear to those skilled in the present art willstill fall within the spirit and scope of the present invention, asdefined in the appended claims.

1. An apparatus of driving a lamp for a display device, the apparatus comprising: an inverter including a transformer for applying a lamp drive voltage to a lamp for turning on or off the lamp and a voltage sensor sensing the lamp drive voltage; a lamp current sensor sensing a current flowing in the lamp and output a feedback signal having a magnitude depending on the sensed current; and an inverter controller comparing a dimming control signal from an external device with the feedback signal and controlling the inverter based on the comparison, wherein the inverter adjusts a turns ratio of the transformer in accordance with the sensed lamp drive voltage.
 2. The apparatus of claim 1, wherein the turns ratio of the transformer is adjusted to a first value when the sensed lamp drive voltage is higher than a predetermined voltage, and to a second value lower than the first value when the sensed lamp drive voltage is less than the predetermined voltage.
 3. The apparatus of claim of 1, wherein the inverter controller operates depending on an enable signal having a plurality of states from an external device, the inverter further comprises a differential circuit supplied with the enable signal, and the turns ratio of the transformer has the first value when the enable signal is in the first state.
 4. The apparatus of claim 3, wherein the inverter further comprises: a driving operation selector having an output value in accordance with outputs of the inverter controller and the differential circuit, a driving unit adjusting the turns ratio of the transformer based on the outputs of the driving operation selector and the inverter controller; and a voltage divider connected between the transformer and the voltage sensor, and making an output voltage of the transformer resonate and dividing the output voltage.
 5. The apparatus of claim 4, wherein the inverter controller outputs a first signal and a second signal, and the driving operation selector comprises: a first OR gate supplied with the first output signal of the inverter controller; a second OR gate supplied with the first output signal of the inverter controller and the output of the differential circuit; a first AND gate supplied with the second output signal of the inverter controller and the output of the differential circuit; a second AND gate supplied with the second output signal of the inverter controller; and an inverter receiving the output of the differential circuit, and generating an output signal to be applied to the first OR gate and the second AND gate.
 6. The apparatus of claim 5, wherein the inverter controller further outputs a third signal and a fourth signal, the transformer includes a primary coil having an input terminal and first and second terminals and a secondary coil, and the driving unit comprises: a first driving circuit supplied with the third and fourth output signals of the inverter controller and generating an output signal to be applied to the input terminal of the primary coil of the transformer; a second driving circuit supplied with the outputs from the first OR gate and the first AND gate and generating an output signal to be applied to the first output terminal of the primary coil of the transformer; and a third driving circuit supplied with the outputs from the second OR gate and the second AND gate and generating an output signal to be applied to the second output terminal of the primary coil of the transformer.
 7. The apparatus of claim 6, wherein the number of turns for the first output terminal of the transformer is less than the number of turns for the second output terminal of the transformer.
 8. The apparatus of claim 6, wherein the driving operation selector makes the second driving circuit select the first output terminal of the transformer to be activated when the sensed lamp drive voltage is higher than the predetermined voltage, and makes the third driving circuit select the second output terminal of the transformer to be activated when the sensed lamp drive voltage sensor is smaller than the predetermined voltage.
 9. The apparatus of claim 4, wherein the voltage divider includes first and second capacitors connected in series between the transformer and a ground voltage, and a common terminal of the first and the second capacitors is connected to the voltage sensor.
 10. The apparatus of claim 9, wherein the voltage sensor comprises: a diode having an anode connected to the common terminal of the first and the second capacitors and a cathode connected to the differential circuit; a resistor connected between the cathode of the diode and a ground; and a capacitor connected between the cathode of the diode and the ground. 